Well, if you're not targeting anything, it would cast on you, which would be a target technically (Which currently doesn't work). Aside from that, I wonder if you're confusing "No target" with "friendly target". Mind sear was changed in the last patch so that you could cast it on friendly targets, such as a tank, for more effectiveness in hitting every mob needing to be burned with AoE.

I shall die here. Every inch of me shall perish. Every inch, but one. An inch. It is small and it is fragile and it is the only thing in the world worth having. We must never lose it or give it away. We must never let them take it from us.

It tries to self cast with no target, so even though you're not actively targeting something else, the spell targets yourself, so there always is a "target". Problem is, it doens't work when targeting yourself. You get a mind sear icon in your buff area, but nothing happens.

Just tested it ingame. While not having target it does target yourself and plays a channel animation, you do gain the mind sear buff for 4 seconds but it does no damage whatsoever. So its just a bug that does nothing.